The present invention is related to the field of capacitive based fluid sensors used to measure fluid levels and a dielectric strength of the fluids using a ratiometric algorithm.
Operation of capacitive based fluid sensors is based upon a measurable change in capacitance of the sensor caused by a difference in the dielectric strength of air as compared to the dielectric strength of a fluid or oil being measured. As fluid levels rise and fall between vertical plates of the sensor, the effective dielectric strength between the plates changes, resulting in an increase or decrease in total capacitance of the sensor. Since the total capacitance is proportional to a product of the dielectric strength and the area of the plates, then given a known constant dielectric strength of the fluid or oil being measured allows the depth of the plates covered by the fluid or oil to be calculated. If the sensor is totally submerged in the fluid, then the measured capacitance can be used to calculate the actual dielectric strength of the fluid.
Capacitive based fluid sensors have found practical applications in engines, transmissions and gear boxes for machines in which the level and quality of the fluid or oil is crucial to the operation and wear of the moving parts. As level sensors, the capacitive based fluid sensors provide a real time indication of the amount of fluid or oil available. The rate at which the fluid or oil level changes can be used to distinguish between normal operational losses and a leak in the system. If the fluid or oil level drops below a predetermined level, automatic alarms or self-preservation routines can be implemented in the machinery to prevent severe damage.
When used to measure dielectric strength, the capacitive based fluid sensors can provide an indication when the fluid or oil is losing its effectiveness, and may indicate a failure in the system. Thermal breakdown of the oil over time generally results in an increase in the dielectric strength of the oil. A combination of time and change in the measured dielectric strength can be used to determine when the oil requires to be changed. Premature or sudden changes in the dielectric strength of the oil can indicate the presence of impurities in the oil. This may indicate severe problems such as engine coolant or fuel leaking around a faulty seal or gasket.
The basic design of many capacitive based fluid sensors limit their accuracy when measuring fluid levels and dielectric strengths. For example, many designs assume a constant dielectric strength when measuring fluid level. If the dielectric strength of the fluid changes with temperature and time, or if a different brand of fluid having a different dielectric strength is substituted for the original fluid, then the measured fluid level changes even though the actual fluid level remains constant. In another example, some sensor designs do not compensate for the effects of temperature on the fluid""s characteristics. The dielectric strength of many fluids is dependent upon the temperature of the fluid. As fluid temperature increases, the dielectric strength increases resulting in a higher measured capacitance. Further, the volume of the fluid may also be dependent upon the temperature. When the fluid is confined in a container, a thermally induced increase in volume will result in an increase in the fluid level in the container. In yet another example, many sensor designs cannot account for the geometry of the container holding the fluid. Many containers have non-uniform shapes causing a non-linear relationship between the level of the fluid and the actual volume of fluid present in the container.
Some capacitive based fluid sensors utilize two active capacitors to mitigate the effects of changing dielectric strength on the accuracy of the fluid level measurements. U.S. Pat. No. 5,929,754 issued to Park et al. on Jul. 27, 1999 is an example of such a dual capacitor sensor. Park""s sensor has a level/deterioration (dielectric) sensing capacitor plus a reference capacitor. The reference capacitor is disposed near the bottom of the sensor so that it is fully submerged in the oil during normal operation. Electronics embedded within the sensor housing use the reference capacitor to partially compensate for variations in dielectric strength over time, for dielectric strength variations between different oil brands, and for geometric changes in the sensor material due to thermal expansion. A temperature sensor is included in the design to help compensate for temperature changes.
The electronics in the Park sensor measure the capacitance of the level/deterioration capacitor and the reference capacitor and output a voltage that is proportional to the difference between the two capacitances divided by the sum of the two capacitances. Trim resistors and other fixed capacitors are used to adjust the output voltage depending upon the use of the sensor for measuring level or oil deterioration. In one embodiment, the electronics toggle the two capacitors between two sets of electronic circuits, one electronic circuit is trimmed for level, the other electronic circuit is trimmed for deterioration. One limitation of this arrangement is that an accurate deterioration (dielectric) measurement can only be made while the level/deterioration capacitor is completely submerged in the oil, and useful level measurements require the level/deterioration capacitor to be only partially submerged.
The present invention is a capacitive based fluid sensor that is capable of generating output signals proportional to a fluid or oil level above a predetermined level and a dielectric strength of the fluid or oil within a container substantially simultaneously. (From this point forward, the fluid or oil will be referred to only as an oil.) The sensor has a compensator capacitor that is disposed within the container and fully submerged within the oil. A linear capacitor of the sensor is disposed within the container so that it initially engages the oil when the oil is at a predetermined level. Both capacitors have openings that allow the oil to flow between capacitive plates of the capacitors. An electronic circuit is electrically connected to both capacitors to measure their respective capacitances. The electronic circuit calculates a dielectric strength based upon the capacitance of the compensator capacitor and an oil level based upon the capacitance of both capacitors. The calculations are performed substantially simultaneously. A temperature sensor may also be positioned to provide an oil temperature signal to the electronic circuit for use in the calculations. An engine speed signal may be provided to the electronic circuit from an external source for use in the calculations.
The capacitors are formed as coaxially cylindrical plates. An inner surface of a hollow housing forms an outer capacitive plate common to both the linear capacitor and the compensator capacitor. A first inner capacitive plate disposed adjacent to the outer capacitive plate completes the linear capacitor. A second inner capacitive plate disposed adjacent the outer capacitive plate completes the compensator capacitor. The first and second inner capacitive plates are held in position by a circuit board assembly that runs the length of the housing. The circuit board assembly is held in position by guides disposed on the inner surface of the housing. In an alternative embodiment, the first inner capacitive plate may include circumferential groves to assist the oil in wetting the plate in predetermined increments.
The housing has one or more openings of suitable size to allow the oil to enter the hollow interior of the housing. A curved section may be included in the housing to allow the sensor to fit into tight locations.
In one embodiment, the electronic circuit includes two oscillators, with one oscillator connected to each of the linear capacitor and the compensator capacitor respectively. The frequency of the oscillators are inversely proportional to the capacitance of the respective capacitors. An advantage of using one oscillator for each capacitor is that both capacitor/oscillator combinations are electrically isolated from each other. A failure of one capacitor/oscillator combination does not necessarily disrupt operation of the other capacitor/oscillator combination. A processor, generally a microprocessor or microcontroller, connected to the outputs of the two oscillators calculates the dielectric strength and oil level. Programming pins connected to the processor, and a memory may be used to load calibration and other characteristics into the electronic circuit for use in the calculations. One or more digital to analog converters may be connected to the processor to convert the dielectric strength and oil level from digital form to analog form. Alternatively, the dielectric strength and oil level may be output in parallel or serial digital form.
Calculation of the oil level and dielectric strength are based upon a plot of the sensor""s characteristics curves using the frequency associated with the linear capacitor (called a first frequency) as the X-axis and the frequency associated with the compensator capacitor (called a second frequency) as the Y-axis. Each curve of the sensor""s characteristic curves intersect at a focal point where the dielectric strength has a value of one. Near this focal point the individual oil level curves are approximately straight lines, each with a unique slope. Each point of the characteristic curve represents an oil level and a dielectric strength. After measuring the first frequency and the second frequency, then the oil level and dielectric strength can be determined from the characteristic curves.
In an alternative embodiment, the oil level calculation determines a difference between the focal point and the first frequency to produce a denominator, and a difference between the focal point and the second frequency to produce a numerator. Calculating a ratio of the numerator to the denominator produces a measured slope that indicates the oil level.
In another alternative embodiment, the dielectric strength calculation is based upon the difference between the second frequency and a known characteristic of a known dielectric strength. A scale factor and offset may be included in the calculation as necessary.
Diagnostics may be performed by the processor to detect, signal and compensate for failures of the capacitors, wiring and some of the electronics. One error detection is accomplished by checking the first frequency and the second frequency against valid frequency bands. A second error detection is accomplished by checking the denominator for a zero value prior to calculating the ratio of the numerator to the denominator. A third error detection is accomplished by checking the calculated ratio against a valid ratio range. If any one or more errors are detected, an error signal may be generated by the processor. Error detection filtering may be applied to minimize spurious errors.
Accordingly, it is an object of the present invention to provide a capacitive based oil sensor that is capable of measuring and reporting the level of the oil and the dielectric strength of the oil substantially simultaneously.
Another object of the present invention is to provide a method of calculating the oil level and dielectric strength using a ratiometeric algorithm.